Cells of D. discoideum mutant Fr-17 were allowed to form multicellular aggregates and develop undisturbed through 12 h (out of 18-required for terminal morphogenesis and cytodifferentiation). Then the cells were disaggregated and redeposited at densities so low as to preclude further sustained cell contacts and were incubated in the presence of certain diffusible metabolites. In this condition they transformed into spores and stalk cells with normal timing and, in the case of the spores, in proportions approaching those observed in undisturbed fruiting bodies. In contrast, mutant cells dispersed from aggregates at earlier stages or wild type cells dispersed from aggregates at any stage, remained as amoebae under the same conditions. The completion of cytodifferentiation by the isolated cells was found to require threshold concentrations of diffusible, dialysable metabolites. A part of this requirement could be satisfied by addition of 10 mM NH4Cl particularly in conjunction with an amino acid mixture. At least one metabolite, however, had to be supplied by feeder cells separated from the test cells by a dialysis membrane or by increasing the population density of the test cells themselves.